1. Field of the Invention
The invention relates to the field of digital optical disk recording systems and more particularly to their apparatus for recovering prerecorded clock signals on readback of data, and in particular, their phase lock loop circuits.
2. Brief Description of the Prior Art
Some digital optical recording systems have a prerecorded clock in the groove or track in which the digital information is also recorded. The prerecorded clock signal is intended to be used both for recording and reading data back. However, when data is recorded, it erases portions of the prerecorded clock when the holes representative of the data are burned or melted into the recording surface.
In optical systems where a reflected laser beam is the read back signal, the inversion of the read signal for data holes is a relatively high amplitude and long lasting unipotential signal. Holes are typically centered on a zero crossing of the clock signal, but may extend over more than one entire clock period for a single hole, and more than one hole may be written consecutively. Moreover, due to limitations in the optical system wherein the laser beam spot size is itself nearly the same size as the hole it is attempting to detect, the signal due to the hole spreads with a significant amplitude one to two clock periods beyond its edge.
The substitution of a relatively high amplitude, unipotential data signal for the prerecorded clock causes obvious problems for phase lock loop circuits attempting to lock onto the prerecorded clock signal on readback of data. Phase lock loop circuits depend upon phase detectors coupled to a uniformly varying signal. The inclusion of a large and long lasting unipotential signal in its input will cause such a detector to drift to an undesirable degree
The typical phase lock loop circuit of the prior art optical recorder has included a bandpass filter just prior to its input to filter out the data signals. These bandpass filters are effective to a degree because the data is recorded in a code which has a null in its power fequency spectrum at the frequency of the prerecorded clock. However, the bandpass filter does not affect the unipotential nature of the signal passed through, and therefore does not completely solve the problem of phase lock loop drift.